Observations of anomalously strong mode-2 internal solitary waves in the central Andaman sea by a mooring system

During the period from July 2016 to July 2018, a mooring system deployed in the central Andaman Sea recorded a significant number of highly intense mode-2 internal solitary waves (ISWs), with wave-induced current strengths comparable to those of local mode-1 ISWs. Distinct propagation characteristics and seasonal variations are uncovered, with mode-2 ISWs being identified as primarily propagating eastward and exhibiting a notable frequency peak from November to March. A significant correlation is established between the occurrence of mode-2 ISWs and higher-mode internal tides (ITs), particularly characterized by a high correlation with the third Empirical Orthogonal Function (EOF) mode. Combined with satellite remote sensing images, it is further confirmed that mode-2 ISWs are generated by the nonlinear steepening of ITs and propagate over long distances through resonance with higher-mode ITs. In contrast, our findings suggest that mode-1 ISWs are predominantly generated by the Lee-wave mechanism, especially south of the Ten Degree Channel, and typically propagate northeastward. This study underscores the complex interplay of ocean stratification and seabed topography in the genesis and propagation of ISWs. •A two-year long in-situ observation has been conducted on the strong mode-2 internal solitary waves in the central Andaman Sea.•The mode-2 internal solitary waves are primarily generated by the nonlinear steepening of internal tides in the Ten Degree Channel, while most mode-1 internal solitary waves originate south of the Ten Degree Channel, produced by the Lee-wave mechanism.•The internal solitary waves of both modes occur more frequently from November to March and less so from May to August. This is primarily attributed to the local oceanic stratification.•The resonant coupling between the mode-2 internal solitary waves and the higher-mode internal tides is the main reason for the stable propagation of the mode-2 internal solitary waves.